SPYDOR wrote:Depending on the material and/or coating of the exhaust tip you ideally want some strong cleaners and a scouring pad or some steel-wool if it is bad as you need to get rid of the build up before you start polishing the exhaust
Something easily obtainable and cheap... such as trisodium phosphate will remove most carbon residues

8 tablespoons to 5l of water should be the right proportions to make a fairly strong cleaner IIRC, some people bump it with some bleach too (approx. 250ml). Just don't get it on your skin, eyes or clothes as it's pretty alkaline and it will do damage

I don't want the blame for anything going wrong, it is a tried and tested method though.